The field of the invention is that of telecommunication satellites. To be more precise, the invention concerns onboard payloads that comprise a plurality of transponders capable of receiving microwave signals from the Earth and transmitting microwave signals to the Earth.
Most existing telecommunication satellites are "transparent" as far as the user is concerned. In this case the resources of the payload are shared between a plurality of separate channels that can be assigned to users on the ground, either at the rate of one channel per user, at least during its use, or at the rate of one channel shared between several users, for example using a time-division multiple access (TDMA) frame or code division multiple access (CDMA) spread spectrum transmission.
A channel is conventionally defined by a center frequency and a bandwidth about that frequency. The transmit frequency is usually different from the receive frequency for a given channel, and there is therefore at least one frequency conversion between the signals received from the Earth and the signals transmitted to the Earth. For a Ku band transponder, for example, the uplinks are at (reception) frequencies around 14 GHz and the downlinks are at frequencies around 12 GHz.
As changes in the satellite telecommunications market track changes in technology, payloads are including increasingly large numbers of transponders, in order to serve a greater number of users simultaneously.
Also, use of the radio frequency spectrum being subject to the pressure of ever increasing demand, payloads must be designed to carry the maximum number of channels simultaneously over virtually all of the frequency band allocated by the international agencies. A consequence of this trend is an increase in the number of different frequency conversions required on the satellite for the latter to fulfil its mission.
For example, to be able to use four of the five 250 MHz transmit-receive sub-bands allocated by the ITU in the Ku band (each sub-band can carry three standard channels with a bandwidth of 72 MHz), six different conversion frequencies are required instead of the two needed in payloads of a previous generation. These multiple frequency changes within the same unit can give rise to a multiplicity of mixing spuriae that can in some cases lead to an unacceptable level of intermodulation between channels and in other cases to channels being "blocked" by interference due to these spuriae.
In a standard architecture several channels are subjected to frequency conversion conjointly, using the same local oscillator signal and, more importantly, the same mixer within the same frequency converter. The signals in the various channels that are transposed conjointly can have very different amplitudes, with differences up to 15 dB, which is important for protecting them against the mixing spuriae mentioned above.
Extrapolating an architecture of the previous generation is not satisfactory since the mass, the weight and the complexity of the standard architecture become prohibitive, as can be seen from FIGS. 1A and 1B.
FIG. 1A shows a first part of one example of a standard payload architecture for a large number of channels (24 channels in the example given, 12 with each polarization XPOL, YPOL). The figure shows the components of the payload for one polarization only, and parallel channels are provided for the cross-polarization YPOL starting from the frequency converter blocks DC1 . . . DC8.
In very broad terms, a payload of this kind comprises, starting from the receive antenna(s): a low noise amplifier block (Fin, LNA); a block of 16 preselector filters (F1, F2, . . . , F15, F16) to divide the received signals into frequency bands 250 MHz wide; eight blocks (DC1, . . . , DC8) each of three frequency converters, representing a total of 24 converters, combined in a 3/2 redundancy ring; a second block of 24 channel filters that form a receive multiplexing block (also known as an input multiplexer IMUX); and a second 16/12 redundancy ring.
As mentioned above, the figure shows only the components of the payload for one polarization (XPOL), and parallel channels are provided for the cross-polarization YPOL starting from the frequency converter blocks DC1 . . . DC8.
Note that each of the eight frequency converter blocks (DC1, . . . , DC8) is fed by a local oscillator that generates the conversion frequency. These frequency converters are shown in more detail in FIG. 3, which corresponds to part of FIG. 1A. Each converter (DC1, DC2) processes a 250 MHz band comprising three contiguous 72 MHz channels. These channels are selected by the 250 MHz bandwidth preselector filter(s) (F1, F2, F3, F4, . . . ) on the input side of the converter. In the example shown in FIG. 1A there are six different conversion frequencies for the eight blocks of converters, and therefore six local oscillators (LO1, LO2, . . . , LO6).
FIG. 3 shows two frequency converters (D1, D2) fed from a single local oscillator LO1. FIG. 1A shows that the same arrangement applies to the converters DC3, DC4 which are fed from a single local oscillator LO2. This is because there are two receive antennas ANT(R) and ANT(T/R) that supply signals in the same 250 MHz sub-band and which can therefore be converted by the same conversion frequency. FIG. 1A shows that for each 250 MHz sub-band only one of the two antennas is selected by switches S on the output side of the input multiplex filters IMUX to feed the channel amplifiers shown in FIG. 1B.
FIG. 3 also shows how the redundancy of the frequency converters is achieved by a standard architecture providing three identical frequency conversion channels for a sub-band to be treated by each block of converters DC1, DC2, . . . with a C2/3 redundancy ring at the input and a C3/2 redundancy ring at the output of each converter unit. One output of each C3/2 redundancy ring supplies a signal corresponding to the polarization X processed by the circuit shown in FIGS. 1A and 1B; the other output corresponds to the polarization Y for which there is a similar circuit that is not shown in order to keep the diagram simple.
All these components are connected in series in the conventional way via coupling and signal routing means comprising, for example, redundant rings (C0, C1, C2, C2/3, C3/2), waveguides (no reference number), 3 dB couplers, switches S, et cetera, to enable signals to be routed correctly and to confer some ruggedness on the payload in the event of failure of a number of critical components of the system, by implementing redundancy in respect of such components, together with switching or coupling means for bringing them into use as necessary.
FIG. 1A stops at the 12/16 redundancy ring and its 16 outputs. The diagram is sectioned at this point, as shown by the arrows A-A', which link to the arrows with the same labels in FIG. 1B, this description of the prior art continuing with a description of the second part of one example of a standard payload architecture for a large number of channels.
Thus, starting from the first 12/16 redundancy ring in FIG. 1A, there are 16 variable gain channel amplifiers CAMP1, CAMP2, . . . , CAMP16 the outputs of which are connected via series-connected noise reduction filters NRF1, NRF2, . . . , NRF16 to the inputs of respective power amplifiers HPA1, HPA2, . . . , HPA16. The signals at the outputs of these power amplifiers are then fed to 18 channel filters OMUX via a second 16/12 redundancy ring. In the example shown, six of these filters OMUX are fed directly from the six outputs of the 16/12 redundancy ring; the other 12 filters OMUX are fed from six other outputs of the 16/12 redundancy ring via six switches S so that six of the 12 OMUX filters can be fed. The signals are then routed to the transmit antenna(s) ANT(T) or transmit/receive antenna(s) ANT(T/R), possibly via other bandpass filters (DI1, TX, . . . ).
This standard architecture causes various problems when used for a payload including a large number of channels and a large number of frequency conversions, some of which are outlined below: a multiplicity of mixing spuriae; a high level of crosstalk; channels blocked by interference; amplification of a plurality of channels at different levels in the same mixer, the operation of which can be optimized for only one of the channels, unless performance is compromised. Moreover, this architecture is not optimized for an implementation with a high level of integration of its various functional components.